The error message doesn’t indicate that there is a problem with the file or that any content is lost, but it isn’t something to ignore. Scrivener autosaves regularly, and the message indicates that at the time the autosave ran, Scrivener was not able to save changes to the binder file. One cause of this is that some other software on your system had the file locked, preventing Scrivener writing to it, and then later that lock was released and Scrivener was able to save the changes on the next try. It’s likely though that you’ll run into the issue again, if this is your situation, so you’ll want to take a look at what software you have running that might be accessing Scrivener’s files and set it to ignore the location where you store your projects. Other software modifying your project files could cause other problems besides just the temporary save warnings.
Memeo Backup (also called WD Anywhere on Western Digital drives) is known to cause this problem, but check on any backup or syncing software you may be using. If you exclude your Scrivener projects from the files that are regularly backed up, be sure that your automatic backups in Scrivener are saving to a location that is still backed up externally, or that you’re regularly saving your own backups to another drive.